Electrolytic polishing of stainless steel



June 1, 1948. A. FEILD 2,442,592

ELEc'rRoLY'nc rousnme or s'mnwss swam.

Original Filed Sept. :50, 1942 INVENTOR. IILEXANDER L. F5140,

part will be Patented Intel, 1948 ELECTBOLYTIO-POHSHING STAINLESS STEELAlexander L. Felld, Baltimore,

assignments, to The Ameri meme MIL, assignor, by can Rolling MillCompany, Mlddletown, Ohio, a corporation of Ohio Original applicationSeptember 80, 1942, Serial RBSSU ED cem r e o. 1 5 Claims. (01.204-140)My application is a division of my copending application Serial No.460,296 of September 30, 1942, entitled Electrolytic polishing ofstainless steel and the invention relates to stainless steel and moreespecially to a process and apparatus for electrolytically polishing thesame.

An object of my invention is the provision of a simple, direct andthoroughly reliable process of electrolytically polishing stainlesssteel. using an alternating electrolyzing current and low currentdensities.

Another object of my invention is the provision of apparatus forelectrolytically polishing stainless steel, which apparatus includes acompact assembly of equipment, which is installed and operated with aminimum of technical advice and skill, which is successfully energizedfrom ordinary alternating current service lines, and which in operationis thoroughly eiilcient and reliable in achieving highly polishedstainless steel products.

Other objects in part will be obvious and in pointed out hereinafter.

The invention accordingly consists in the combination of'features andarrangement of parts and in the combination of steps and the relation ofeach of the same to one or more of the others as described herein, thescope of th application of which is indicated in the following claims.-

In the accompanying drawing there is illustrated schematically anelectrolytic polishing system for carrying out the practice of myinvention.

As conducive to a clearer understanding of certain features of myinvention, it may be noted at this point that stainless steel is definedas low-carbon steel comprising to 35% or more chromium, with or withoutnickel, and with or without supplemental additions of manganese,silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium,titanium, sulphur, and

the like, for special purposes, and a-balance substantially of iron.

It may be noted further that stainless steel is characterized inunpolished grayish appearance. With proper polishing treatment, however,such steel is finished to possess a brilliant, mirror-like surface.There are, for example, certain prior art methods of polishing stainlesssteel which involve mechanical polishcondition by a dull,

, their originally intended use.

ing steps, such as abrading, boiling, and the like. These methods,however, aside from being tedious and time-consuming, require skilledlabor and frequent replacement of polishing equipment. They do not giveuniformly polished products and are especially'unsatisfactory wherepeculiar or intricate metal surface contours are encountered.

More recently, processes of finishing stainless steel surfaces have beendevelopedwhich involve electrolytic polishing. As compared with thestill widely used grinding, boiling and like mechanical polishingmethods, the eelctrolytic polishing methods are far more simple, andmore-rapid and economical of performance, Electrolytically polishedproducts, moreover, possess greater brilliance and improved beauty,there being a greater evenness and uniformity of finish.

The art of eelctrolytic polishing, on the other hand, being relativelynew in the stainless steel industries, still presents a number ofdifficulties. In this connection, it has been common practice in theheretofore known electrolytic polishing processes to employ directcurrent electrical energy which, where only alternating current isreadily available, represents a problem. It becomes necessary to installexpensive batteries or a motor-generator set to supply direct current inthe polishing system and thus, in effect, the unit cost of polishedsteel produced is increased. Again, certain of these heretofore knownprocesses frequently yield products which, because of being etched ordiscolored, are unsuitable for Other such processes are not adapted forrapid, large scale production of polished stainless steel products andthus possess no great industrial utility.

An outstanding object of my invention, accordingly, is the provision ofan economical, industrially practical process of electrolyticallypolishing. stainless steel wherein the work is evenly polished in anelectrolyte solution by means of alternating electric current, whichprocess is rapid of performance, and which gives highly polishedproducts free of discoloration and free of etched or pitted areas.

Referring now more particularly to the practice of m invention, Ielectrolytically polish articles or products fashioned of stainlesssteel in the form of sheet, strip. plate. wire, bars, rods, rounds, andthe like, using the article or product as one or more alternatingcurrent electrodes of an electrolyticbath. The bath itself consists ofconcentrated nitric acid, with or without a substantial amount ofconcentrated acetic acid.

As illustrative of the practice of my invention. attention beingdirected to the drawing, I provide in a suitable container 8, such as anopen tank having a lead or other acid-resistant lining, an electrolytesolution l containing, by volume. for example, 80% to 40% concentratednitric acid (density 1.40-1.42) and 20% to 60% concentrated acetic acid(glacial acetic acid density 1.048). Within the volume ranges specified.I prefer most especially to employ an acid solution containingapproximately 70% concentrated nitric acid and about 30% concentratedacetic acid. Although the presence of concentrated acetic acid in thebath is preferred, the acetic acid serving as a buffer, it is within thescope of my invention to use concentrated nitric acid alone. In sodoing, I employ by volume of electrolyte about 80% or more concentratednitric acid and a balance principally of water.

Alternating current electrical energy conveniently is supplied to thebath my way of a stepdown transformer l2 connected across a suitablesource of alternating current supply ll. Opposite ends of thetransfonner secondary winding l2a are connected respectively withsuitably spaced immersed electrodes l3 and ll of the bath, as byconductors Ba and Ma. The elec-- trode i3 is, for example, a plate oflead or the lining of tank 9, while the electrode H comprises astainless steel work piece to be polished.

Upon energization of the transformer, alternating current coursesthrough the concentrated acid electrolyte between the immersedelectrodes and polishing of the stainless steel work begins promptly andproceeds rapidly. The polishing operation preferably is performed usinga low current densit ranging approximately from A to 4 amperes persquare inch of exposed surface. Such density, however, may be increased,depending on other working conditions up to 12 or more amperes persquare inch.

During the polishing operation a considerable amount of heat develops inthe electrolytic bath. There is a tendency for the temperature of thebath to increase. This tendency is counteracted by providing suitablecooling means in or adjacent the electrolyte, such as a refrigeratorcoil generally indicated at l5; or by stirring the electrolyte. UsuallyI maintain an electrolyte temperature which ranges from 12 C. to +15 C.,

for it is within this range that I achieve best polishing results and,at the same time. incidentally avoid excessive evaporation of theelectrolyte. I find, where solution temperatures extend upwardly ofapproximately 25 C., the immersedstainless steel work is exposedobjectionably to attack and is etched or pitted and, thus. such bathtemperatures are to be avoided. The temperature range. therefor, is inevery sense critical to the successful operation of my process.

A satisfactory polish ordinarily requires about three minutes time. Thework then is withdrawn from the bath and disconnected from the source ofsupply. It then is rinsed thoroughly in clean water after which it isready for use or sale. It possesses a mirror-like surface which isevenly polished and highly attractive.

In my process, the actual polishing of the stainless steel is done onthe positive half-cycle of the alternating current. The opposite ornegative half-cycle of current surprisingly does not de- 4 stroy thepolish. By virtue of this phenomenon the stainless steel work itself mayrepresent either electrode of my polishing system. and be polished withequally as good results. In fact, both elec-' trodes may at the sametime be represented by individual stainless steel work pieces. Polishingof the work then is successfully achieved in a thoroughly eflicient andpractical manner at both electrodes.

My electrolyte polishing process is successfully practiced on stainlesssteel articles and products of widely differing quality and consistentlygives uniformly polished stainless steel surfaces even where surfaces ofintricate contour are encountered. Where the metal under treatmentpossesses a hammered surface, such as is often the case of ornamentaltrim, hardware, and objects of art including bowls and urns, an evenlustrous polish nevertheless is obtained. Other surfaces of intricatecontour such as those encountered on stainless steel grilles, trays, andthe like, fabricated by the welding of wire, strip or other convertedforms, also are polished successfully. The polished surfaces achievedare free of pits or blemishes and the welded portions likewise are freeof burns, thus resisting corrosion and maintaining their attractivenessover a long period of time.

The electrolytic polishing process which I provide is, moreover, rapidand economical of performance and is quite adaptable to continuous ormass production of polished stainless steel products of outstandingquality. The equipment needed is cheap and readily available, requireslittle space for installation, and after installation is easily operatedand maintained.

Thus, it will be seen that there is provided in this invention a processof electrolytically polishing stainless steel in which the variousobjects hereinbefore noted together with many thoroughly practicaladvantages are successfully achieved. It will be noted that the processgives in a simple and economical manner, evenly polished stainless steelproducts which are free of pits and other surface flaws, and whichpossess outstanding lustrous beauty. It will benoted further that theprocess is performed efllciently and reliably through the use ofapparatus energized by the readily available alternating electriccurrent.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiments hereinbefore set forth, it is tobe understood that all matter described herein or shown in theaccomanying drawing is to be interpreted as illustrative and not as alimitation.

I claim:

1. The art of electro-polishing a stainless steel product whichcomprises, immersing the product to be polished in a bath containing byvolume or more of concentrated aqueous nitric acid (sp. gr. 1.40-1.42)and the remainder being water, and while maintaining the bath at atemperature below 25 C., passing only alternating current through saidbath between the product and a cooperating electrode immersed therein,the current density being at least amperes per square inch of productsurface.

2. The art of electro-polishing a stainless steel product whichcomprises, immersing the product to be polished in a bath containing byvolume 80% or more of concentrated aqueous nitric acid (sp. gr.1.40-1.42) and the remainder being water, and while maintaining the bathat a temperature -12 C. to +15 0., passing only alternating currentthrough said bath between the product and a cooperating electrodeimmersed therein, the current density being at least /2 amperes persquare inch of product surface.

3. The art or electro-polishing a chromiumnickel stainless steel productwhich comprises, immersing the product to be polished in a bathcontaining by volume 80% or more 01 concentrated aqueous nitric acid(sp. gr. 1.40-1.42) and the remainder being water, and whilemaintaintemperature below 25 0., passing only alternating currentthrough said bath between the product and a cooperating electrodeimmersed therein, the current density being at least' /i to "12 amperesper square inch of product surface.

4. The art of electro-polishinl stainless steel products whichcomprises, immersing two stainless steel products to be polished in abath containing by volume 80% or more or concentrated acid (sp. gr.1.40-1.42) and the remainder water, and passing only alternating saidbath and between said products at a current density of per square inchor surface of each product while at least amperes 26 maintaining thebath temperature below C.

5. The art of electric-polishing a stainless steel product whichcomprises, immersing the product to be polished in a bath containing byvolume concentrated aqueous nitric acid (sp. gr.

1.40-1.42) and the remainder water, and while REFERENCES CITED Thefollowing references are of record in the tile of this patent:

UNITED STATES PATENTS Great Britain

